Haml is great for many cases, but it's terrible when the ratio of text to embedded code gets large. Chris Eppstein, one of the developers of Haml, made a blog post to this effect last year:
http://chriseppstein.github.com/blog/2010/02/08/haml-sucks-f...
As a result, though I love Haml, I think ERb is a more sensible default.